New Mn and\nV‑rich Phosphate Fluoride Obtained\nby Topochemical Reaction for Na-ion Batteries Positive Electrode

A new positive electrode material, Na<sub>2.85</sub>Mn<sub>0.4</sub>V<sub>1.6</sub>(PO<sub>4</sub>)<sub>2</sub>F<sub>2.4</sub>O<sub>0.6</sub>, is synthesized via a topochemical reaction in an\nionic liquid medium,\nstarting with a tailored precursor Mn<sub>0.2</sub>(VO)<sub>0.8</sub>PO<sub>4</sub>·2H<sub>2</sub>O. Its structural and chemical\ncharacterization was conducted using a comprehensive set of techniques\nincluding X-ray diffraction, X-ray absorption, and electron paramagnetic\nresonance spectroscopies, as well as inductively coupled plasma optical\nemission spectroscopy and electron probe microanalysis. These analyses\nnot only allowed to determine the composition and structure but also\nshed light on the synthesis reaction mechanism. The resulting active\nmaterial exhibits promising electrochemical performance, delivering\na high capacity of 108 mA h/g at a rate of C/20 with an average potential\nof 3.75 V vs Na<sup>+</sup>/Na. Even at a higher rate of 1C, a specific\ncapacity of 90 mA h/g is maintained and an excellent capacity retention\nof 94% is demonstrated after 200 cycles at C/5. In addition, XAS analysis\nconducted on materials recovered at different states of charge reveals\nthe redox activity of both manganese and vanadium centers. More generally,\nthis work showcases the feasibility of synthesizing stable Na-deficient\npolyanionic phases within the Na<sub><i>z</i></sub>Mn<sub><i>x</i></sub>V<sub>2–<i>x</i></sub>(PO<sub>4</sub>)<sub>2</sub>F<sub>3–<i>y</i></sub>O<sub><i>y</i></sub> (0 ≤ <i>x</i>, <i>y</i> ≤ 2, and <i>z</i> ≤ 3.6) material\nfamily.